There was this recent interview with Dijana, Novak's mom, where she revealed that they didn't put as much money and dedication into Marko's career. So maybe that's it.
There were situations when we couldn't give everything to all three of them, for example, when Marko was supposed to go further and develop. We didn't have financial possibilities for Novak and Marko. And Novak had already started his way and then everything went on him. And because of that, Marko stagnated a little bit for 1-2 years.t